With the speckled background covered in plain gray, the two patches look the same.
Lighter or darker?
Chubb illusion
Which speckled patch has the stronger contrast?
Neither: they are the same, though the left one looks bolder.
Why? Your eye adjusts to how much contrast is around. Surrounded by strong contrast, the patch looks faint.
Picture: w:User:EyesDude, CC BY-SA 3.0 · Wikipedia(opens in a new tab)